The braking systems installed in vehicles, such as four-wheeled vehicles, include the ones (so-called electric boosters) configured to electrically control the brake fluid pressure generated in master cylinders in order to supply brake fluid to wheel cylinders of the vehicles. These braking systems each have an electric motor which outputs a braking force in accordance with a braking demand signal, and a controller which controls the electric motor (see Patent Literature 1, for example).
According to the braking system described in the Patent Literature 1, when a main power source is decreased in its remaining power, electric power is supplied to a controller from an auxiliary power source. If the remaining power of the main power source is further decreased, an electric motor is also supplied with electric power from the auxiliary power source.